Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for imaging and characterizing a subsurface and more specifically to a method for removing water bottom and/or geology contamination from near-field sensor recordings.
Discussion of the Background
Reflection seismology is one tool in the geophysical exploration arsenal for determining the properties (e.g., an image) of a subsurface layer in the earth, which information is especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source (e.g., air gun, vibratory element, etc.) that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections and thus, to generate an image of the subsurface. These features may be associated with subterranean hydrocarbon deposits and the generated image may indicate, to those skilled in the art, the location of these deposits. Thus, improving the functionality of any component (source or receiver or computer that generates the image) of the seismic acquisition system results in a better image of the subsurface, and consequently, a higher likelihood of finding the deposits.
For marine applications, commonly used seismic sources are essentially impulsive (e.g., air guns that hold compressed air that is suddenly allowed to expand). An air gun produces a high amount of acoustic energy over a short time. Such a source is towed by a vessel at a certain depth along a given direction (called inline direction). The acoustic waves from the air gun propagate in all directions. The air gun instantaneously releases large peak acoustic pressure and energy.
Such a source array is illustrated in FIG. 1. FIG. 1 shows a generic source array 104 (note that the full configuration of the source array is not shown for simplicity) being towed behind a vessel 101. When the source array is activated, acoustic energy is coupled into the water and transmitted into the earth, where part of the energy is partially reflected back from the ocean bottom 113 and from rock formation interfaces 112 (rock layer that has a change in acoustic impedance). Sensors or receivers 106 used to record the reflected energy include hydrophones, geophones and/or accelerometers. The receivers can be encapsulated in either fluid filled, gel filled or solid streamers 105 that are also towed by vessels at shallow depth.
Returning to the air guns, an air gun stores compressed air and releases it suddenly underwater when fired. The released air forms a bubble (which may be considered spherical), with air pressure inside the bubble initially greatly exceeding the hydrostatic pressure in the surrounding water. The bubble expands, displacing the water and causing a pressure disturbance that travels through the water. As the bubble expands, the pressure decreases, eventually becoming lower than the hydrostatic pressure. When the pressure becomes lower than the hydrostatic pressure, the bubble begins to contract until the pressure inside again becomes larger than the hydrostatic pressure. The process of expansion and contraction may continue through many cycles, thereby generating a pressure (i.e., seismic) wave. The pressure variation generated in the water by a single air gun (which can be measured using a hydrophone or geophone located near the air gun) as a function of time is called the near-field signature and is illustrated in FIG. 2.
A first pressure increase due to the released air is called primary pulse. The primary pulse is followed by a pressure drop known as a ghost arrival. The ghost arrival relates to a reflection of the source energy leaving the source upwards and being reflected downwards from the free surface. Between highest primary pressure and lowest ghost pressure there is a peak pressure variation (P-P). The pulses following the primary and the ghost are known as a bubble pulse train. The pressure difference between the second pair of high and low pressures is a bubble pressure variation Pb-Pb. The time T between pulses is the bubble period.
A source that includes a single air gun is not practical because it does not produce enough energy to penetrate at desired depths under the seafloor, and plural weak oscillations (i.e., the bubble pulse train) following the primary (first) pulse complicates seismic data processing. These problems are overcome by using arrays of air guns (i.e., the source array), which generate a larger amplitude primary pulse and canceling secondary individual pulses by destructive interference.
FIG. 2 represents a situation in which the bubble generated by a single air gun drifts slowly toward the surface, surrounded by water having the hydrostatic pressure constant or slowly varying as the bubble slowly drifts upward. However, when another air gun is fired simultaneously in proximity to the first air gun, the hydrostatic pressure is no longer constant or slowly varying. The bubbles of neighboring guns affect each other.
A source array includes plural individual source elements (e.g., air guns). An individual source element may be a single air gun or a cluster of air guns. Since the dimensions of the source array, including plural individual source elements, are comparable with the generated wave's wavelength, the overall wave generated by the source array is directional, i.e., the shape of the wave, or the source signature varies with the direction until, at a great enough distance, the wave starts having a stable shape. The air guns may all be positioned at the same depth, or may be positioned at a variety of depths. After the shape become stable, the amplitude of the wave decreases inversely proportional to the distance. The region where the signature shape no longer changes significantly with distance is known as the “far-field,” (or where the wavelength of the wave is much smaller than a distance d from the gun to the observation point) in contrast to the “near-field” region (where the wavelength is larger than distance d) where the shape varies. Knowledge of the wave source's far-field signature is desirable in order to extract information about the geological structure generating the detected wave upon receiving the far-field input wave.
An accurate description of a seismic source signature is required in order to carry out a number of functions in seismic data processing. For example, source de-signature, deghosting, zero-phasing or de-bubbling, may be carried out using far-fields derived for a seismic source, which may be one-dimensional (1D), two-dimensional (2D) or three-dimensional (3D). The source signature may or may not include the free surface ghost.
In order to estimate the source array's far-field signature, an equivalent notional signature for each individual source may be calculated for each of the guns using near-field measurements (see e.g., U.S. Pat. No. 4,476,553 incorporated herewith by reference). The equivalent notional signature is a representation of an amplitude due to an individual wave source as a function of time, the source array's far-field signature being a superposition of the notional signatures corresponding to each of the individual sources. In other words, the equivalent notional signature is a tool for representing the contribution of an individual source to the far-field signature of the source array, such that the individual source contribution is decoupled from contributions of other individual wave sources in the source array. It should be understood that the notional sources may contain some energy arising from bubble interactions with other bubbles.
Ziolkowski et al., “The signature of an Air Gun Array: Computation from Near Field Measurements Including Interactions”, Geophysics, Vol. 47, No. 10, pp. 1413-1421 (October 1982) and Parkes et al., “The Signature of an Air Gun Array: Computation from Near-Field Measurements Including Interactions—Practical Considerations, Geophysics, Vol. 48, No. 2, pp. 105-111 (February 1984), proposed the inversion of data obtained from near-field hydrophone (NFH) sensors mounted on the source array in the vicinity of the seismic air-guns, i.e., in the near-field, to obtain the source signature characteristics. Key to this methodology is the definition of notional sources as source signatures in the proximity of individual air guns within the seismic source array, but taking the interactions between the bubbles, which take place when the guns are fired together, into account. These notional sources are used to generate the source array signatures as desired.
An alternative approach where NFH data is not available is described in WO 2016/083892. In this approach, direct arrival data is used to derive a representation of the source emission.
However, if the water bottom reflection time is less than the required far-field signature length (i.e., the water is shallow), the NFH sensors mounted on the source array will record reflections from the water bottom and geology of the subsurface (because of their location and/or sensitivity, the NFH sensors record only shallow geology, i.e., geological features that are not very deep under the water bottom), in addition to the primary signals from the individual airguns and ghost reflections of the airguns from the sea surface. For example, if a 1 second far-field signature is required, then a water depth d corresponding to the 1 s far-field signature is d=vt/2=1500 m/s*1 s/2=750 m, where v is the velocity of sound in water and t is the desired far-field length. The water bottom and geology signals recorded by NFH sensors are smaller in magnitude compared to the primary and ghost signals, nevertheless, if not removed, this reflected data will result in corresponding contamination in the notional sources and far-fields inverted from the NFH data.
For those situations where such contamination is present in the NFH data, but the water bottom and geology are not flat, the contamination may be removed by stacking each NFH recording along the line to give an averaged set of NFH recordings, from which global notional sources and far-fields may be inverted for the line. An example of this approach is given in Ni, Y., Haouam, F., and Siliqi, R., 2015, “Source signature estimation in shallow water surveys,” SEG conference proceedings. In many cases, if shot by shot notional sources and far-fields are required, this method is unsuitable.
An alternative is to extend the approach of Ziolkowski by modeling the water-bottom reflection as part of the NFH inversion as described in Hopperstad, J. F. and Laws, R., 2006, “Attenuation of the seafloor reflection error in shallow water,” EAGE conference proceedings and U.S. Pat. No. 7,440,357. This approach requires accurate information about the timing and reflectivity of the water bottom.
Furthermore, in cases where the water bottom or geology are flat, even the stacked NFH recordings will still contain the reflections from the sea floor and geology, resulting in contaminated results for the global notional sources and far-fields.
Thus, there is a need to develop a method that can remove the contamination from the NFH recordings.